In recent years, a technology to form a thin film transistor by using a semiconductor thin film (film thickness of about several nm to several hundred nm) formed on a substrate having an insulating surface is attracting attentions. A thin film transistor, which is widely applied to electronic devices such as an IC and an electronic optical device, is rapidly developed in particular as a switching element of an image display device.
In general, a liquid crystal display device is known as an image display device using a thin film transistor. An active matrix liquid crystal display device is becoming to be more used which can provide an image in higher resolution as compared to a passive matrix liquid crystal display device. In an active matrix liquid crystal display device, a pixel electrode provided in each of the pixels in matrix is selectively driven to form a display pattern on a display. That is, by applying a voltage between a selected pixel electrode and a counter electrode corresponding to the selected pixel electrode, a liquid crystal layer between the pixel electrode and the counter electrode is optically modulated. This optical modulation is recognized as a display pattern by an observer.
Application range of such an active matrix liquid crystal display device is expanding and demands for a high resolution, high aperture ratio, and high reliability are increasing in accordance with a display size becoming large. At the same time, demands for the improvement of productivity and cost reduction are increasing.
The production cost of a conventional active matrix liquid crystal display device is high since TFTs are formed on a substrate by using a number of photo masks by photolithography technology. In order to improve the productivity and yield, it is considered efficient to reduce the number of steps.